The purpose of screening is not simply to detect disease earlier, but rather to help people live better or longer (i.e., improve health outcomes) because of early detection. Thus, the question we need to ask ourselves in considering a screening situation is not how many early cancers we find but how many people avoid poor health outcomes.
Although many people and their clinicians view the potential benefits of screening as primarily a function of the accuracy (especially the sensitivity) of a screening test, in fact, the factor that most commonly limits the benefit from screening is the treatment. For a screening program to improve health outcomes, it must include a treatment that is not only effective but which is more effective if applied earlier than if applied later. That is, the critical issue with screening is the timing of treatment. If the treatment is not effective at any time, obviously screening is not useful. If treatment is excellent and just as effective for clinically detected cancer as screening-detected cancer, then again early detection by screening is not helpful. Screening is only useful in improving health outcomes when the treatment is effec tive for screening-detected cancer but not clinically detected cancer.
This treatment criterion for a screening program is often misunderstood. The important question is this: where in the natural history of this cancer is the critical point (see following discussion) at which a particular treatment becomes ineffective? Theoretically, at least, many treatments may be effective when a potentially fatal cancer is only a few cells in size. As this cancer grows, however, there comes a point at which treatment is no longer effective in altering its natural history and helping the person to live better or longer. It is the location of this critical point, and especially its relationship with the point of detection by the screening test, that determines the effectiveness of the screening-and-early-treat-ment program. If the critical point is earlier than the point at which the cancer can be detected by screening, then screening cannot be helpful. If the critical point is during the "lead time" produced by the screening test, then screening will be helpful. If the treatment is very effective and the critical point is after the point at which regular, competent medical care would detect the cancer, then screening is not useful because treatment after usual clinical detection is as effective as treatment after screening detection.
When treatment is particularly effective, the critical point for some potentially fatal cancers may be at a far-advanced stage. Even very effective treatments may become ineffective for far-advanced stage cancers. Far advanced stage cancers at diagnosis may occur in several situations: in people who neglect their health; in people without access to regular, competent medical care; or in people without understanding that early signs or symptoms should be evaluated. In the past, for example, some women presented with breast tumors that were the size of a lemon or even an orange. It would be difficult to deny that many such cancers could have been treated more successfully had they been evaluated at an earlier stage. Few women present with such advanced tumors now, at least partly, because most women in this country recognize that breast lumps of any size should be examined by a physician.
The treatment requirement for a screening program is that the treatment must be more effective after detection by screening than after usual clinical detection. It does not require that the treatment be effective for far-advanced stage cancers. One does not need to implement a screening program to prevent the development of far-advanced cancers by helping people understand that new symptoms and signs should be reported to one's physician. This educational effort is different from screening.
The issue of the effectiveness of treatment at different points in the natural history of cancer is made more complex by the marked variation in cancers and individuals, as just discussed. It is not surprising, for example, that early detection and treatment rarely reduce mortality by 100%. For example, in the overviews of the randomized controlled trials of breast cancer screening, mortality is reduced by less than 20%.6 This finding would imply that about 20% of women destined to die of breast cancer have a type of cancer that is better treated earlier than later. The other 80% of women destined to die of breast cancer have a type of cancer for which earlier treatment is not useful. These women may have a particularly malignant form of cancer in which metastasis occurs at an early stage, too early to be detected by screening.
Many other women are detected by breast cancer screening, of course, but these may be women not destined to die of breast cancer. They may have either a less-malignant form of the disease for which later treatment is as effective as earlier treatment or a benign form of cancer that would never have caused major adverse health outcomes even without treatment.
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